Xeroradiographic process for preventing unwanted ion discharge of a charged photoconductor during x-ray imaging thereon

ABSTRACT

This application relates to a xeroradiographic process wherein an insulating membrane having a matte or textured surface, as described herein, is positioned in contact with the charge sensitized surface of the xeroradiographic plate during imaging exposure. The interposition of such a membrane between the X-ray source and the xeroradiographic plate eliminates the adverse effects of ion generation adjacent the charge sensitized surface of the xeroradiographic plate.

United States Patent Inventors William Joseph Kearns Arcadia; Jerry Wayne Hedstrom, San Gabriel, both of, Calif. Appl. No 876,024 Filed Nov. 12, 1969 Patented June 8, 1971 Assignee Xerox Corporation Rochester, N.Y.

XERORADIOGRAPHIC PROCESS FOR PREVENTING UNWANTED ION DISCHARGE OF A CHARGED PHOTOCONDUCTOR DURING X-RAY IMAGING THEREON 22 Claims, No Drawings US. Cl 250/65, 2591495 Int. Cl ..G03g ltd/Q0, G03 g l5/00 Field of Search 250/65 56] References Cited UNlTED STATES PATENTS 2,802,949 8/1957 Lehmann 250/65.2

Primary Examiner-James W. Lawrence Assistant Examiner--C. E. Church Attorneys- Paul M. Enlow, James J. Ralabate, Ronald Zibelli and Joseph I. Hirsch ABSTRACT: This application relates to a xeroradiographic process wherein an insulating membrane having a matte or textured surface, as described herein, is positioned in contact plate.

BACKGROUND OF THE INVENTION This invention relates to xeroradiography and, more particularly, to an improved method of alleviating the adverse effect of air ionization during xeroradiographic imaging.

Xeroradiography, as disclosed in U.S. Pat. No. 2,666,144, is

a process wherein an object is internally examined by subjecting the object to penetrating radiation. In xeroradiography, the xeroradiographic plate exposed to the X-ray pattern usually comprises a metallic backing member having a photoconductive insulating layer or coating, for example, vitreous selenium, on one surface thereof. It is conventional to cover or protect the photoconductive layer from ambient actinic radiation by the use of a so-called dark slide spaced from the photoconductive surface. In an automated xeroradiographic system, a cassette, such as described in application Ser. No. 874,747, filed Nov. 7, 1969, can be utilized to define a light-tight environment during imaging exposure and development. The plate or element is sensitized by applying a uniform electrostatic charge to the photoconductive layer and thereafter the charged plate is exposed to the sensitizing radiation pattern with the object to be examined appropriately interposed between the radiation source and the sensitized plate. Under influence of the X-rays emanating from the source, which are differentially absorbed by different areas of the test object, the layer becomes electrically conductive in those portions reached by the sensitizing radiation, thereby permitting portions of the electrostatic charge thereon to be selectively dissipated. Dissipation of the electrostatic charge is proportional to the amount of radiation absorbed by the test object with greater dissipation occurring in those portions of the coating shaded by less absorptive portions of the object being radiographed. In this manner, an electrostatic latent image of the test object is formed on the photoconductive surface of the xeroradiographic plate. The image may then be made visible with an electroscopic marking material which clings to the electrostatically charged portions of the latent image. Reversal, or negative, prints can also be developed by contacting the latent electrostatic image with marking particles charged to the same polarity. The visible image may be viewed, photographed or transferred to another surface where it may be permanently affixed or otherwise utilized.

When a xeroradiographic exposure is made in the manner described, the latent electrostatic image in many cases is adversely degraded by discharge thereof not associated with the physical characteristics of the test object undergoing examination. A primary cause of this degradation is aii ionization produced in the air space between the surface of the photoconductive coating and the interior surface of the dark slide or cassette adjacent thereto.

In Lehmann US. Pat. No. 2,802,949, ion-caused undercutting or image degradation is stated to be eliminated by excluding ionizable gases, such as air, from adjacent the surface of the sensitized photoconductive layer of the xeroradiographic plate during imaging exposure. One way in which the ionizable gas can be excluded is by providing a thin insulating medium in direct and intimate contact with the charged photoconductive surface. It is stated that intimate contact is necessary to totally exclude the ionizable gas from a position adjacent the photoconductive surface.

Recent experience with the Lehmann technique, in view of increased interest in the use of xeroradiography in the early detection of breast cancer in women or in the enhancement of body extremities, has shown that the Lehmann technique is not totally satisfactory in yielding nonadversely degraded xeroradiographic images. Initially, while it has been found that the Lehmann technique, by use of the insulating materials described therein, can be made to work if the operating conditions are exactly right, and the contact pressure not too great, such cases are rare and, for all practical purposes, impossible to control, especially in an automated xeroradiographic processing system such as described in application Ser. No. 874,834, filed Nov. 7, 1969, assigned to the assignee of the present invention. That is, the insulating materials suggested by Lehmann when positioned in intimate contact with the photoconductive surface during imaging exposure will produce image artifacts which significantly degrade the resultant image such as to make it unsuitable in the field of medical diagnostics. Additionally, Lehmann identifies the problem on Lichtenburg figures which can be formed if the insulating material is not properly applied to, and removed from, the photoconductive surface. Once again, this characteristic makes the Lehmann technique, as described therein, unsuitable for automated xeroradiographic processing systems.

It is, therefore, desirable to have an improved xeroradiographic imaging technique wherein the adverse effect of ioncaused degradation of the latent electrostatic image is eliminated in a manner which is adaptable to automated xeroradiographic examination.

OBJECTS OF THE INVENTION It is, therefore, an object of the present invention to provide an improved xeroradiographic imaging technique.

It is a further object of the present invention to provide an improved xeroradiographic imaging technique adapted for use in automated xeroradiographic processing yet does not adversely affect image quality.

It is a further object of the present invention to provide an improved xeroradiographic technique for creating a latent electrostatic image on the photoconductive surface of a xeroradiographic plate, the technique including the positioning of an insulating medium, having the physical characteristics set forth below, in contact with the photoconductive surface of the xeroradiographic plate during exposure.

It is a further object of the present invention to define the physical characteristics of insulating materials more suited for use in the Lehmann technique in that the use of such materials does not adversely degrade the latent electrostatic image formed during xeroradiographic exposure.

Yet a still further object of the present invention is to define additional insulating materials, and the physical characteristics thereof, which make such materials better suited for use with the Lehmann technique, such that nonadversely degraded images are obtained during xeroradiographic exposure. I

These and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed disclosure.

BRIEF SUMMARY OF THE INVENTION These and still further objects of the present invention are achieved, in accordance therewith, by providing in contact with the xeroradiographic plate during exposure a membrane which does not cause, foster or promote degradation of the latent electrostatic image resulting from X-ray penetration of the test body. The membrane should have a resistivity at least equal to the dark resistivity of the photoconductive material or the bulk resistivity of an insulating coating thereon, and the surface of the membrane most closely adjacent the photoconductive material is one which provides a plurality of micropoint contacts which, if discharge should be caused thereby, the area thereof would be below the limit of the eye's resolving power, such that it would not be discernible during visual'examination of the developed image. In one embodiment, the membrane has an irregular nonflat surface adjacent the photoconductive layer. In a further embodiment, the membrane has a pattern of fibers extending from the bulk of the membrane to the photoconductive surface. Consistent with the other requirements of. the membrane, the fibers can be regularly or randomly spaced, and can be at acute or right angles to the photoconductive surface.

To be fully effective in eliminating image degradation, the surface area of each point contact should be on the order of a circle having a diameter of about 0.001 to about 0.002 inch, i.e., below the limit of the eyes resolving power, such that should the point contact cause discharge of the latent image, such discharge cannot be detected during visual examination of the developed image. The distance between adjacent point contacts and the distance across the cavity defined by adjacent point contacts, the surface of the xeroradiographic plate and the spaced, adjacent surface of the bulk membrane should not be greater than the ionizing mean free path of an X-ray generated electron in air. For all practical imaging purposes, this is on the order of about 0.001 inch to about 0.003 inch. If the aforementioned dimensional characteristics are observed in any particular chosen membrane, the area in direct contact with the xeroradiographic plate will be on the order of about 0.1 percent to about 1 percent of the photoconductive surface. This is in contrast to the materials described in Lehmann wherein it is estimated that no less than 10 percent of the photoconductive surface is in intimate contact with the overlying insulating membrane. Additionally, the point contacts in the Lehmann materials are so close together, and the surface so relatively fiat, that tribocharging can occur, with resultant production of Lichtenburg figures upon stripping of the adjacent surfaces.

During imaging exposure, electrostatic forces will be created which will have a tendency to pull the membrane toward the xeroradiographic plate surface. While the membrane should have a plurality of point contacts spread across the surface of the xeroradiographic plate, there should not be so many contacts, or contacts so closely spaced to the plate surface, as to provide an essentially flat surface adjacent the xeroradiographic plate, such that electrostatic-induced movement of the membrane toward the plate surface will broaden the contact points to cause image discharge which can be visually perceived.

The membrane should be sufficiently thin and have a sufficient uniform internal structure such that it does not differentially absorb X-radiation during the imaging exposure and does not cause the formation of a visible artifact in the form of the membrane texture, superimposed over the test object image. Notwithstanding the thinness of the membrane, it should have sufficient strength to resist broadening of the surface area of the point contacts under compressive stress, either electrostatically-induced or otherwise.

The resistivity of the membrane should be on the order of ll0' ohm-cm, preferably on the order of about 10" 10" ohm-cm. Ideally, the resistivity of the membrane should approximate the dark resistivity of the photoconductive material, or an overlying protective coating thereon; however, higher resistivities can be tolerated because of the numerous air gaps provided between the insulating material and the adjacent surface of the xeroradiographic plate. This range of resistivity is sufficient to prevent lateral charge transfer from the surface of the xeroradiographic plate through a point contact, the bulk of the membrane material and adjacent point contact, Such a transfer would result in a latent image not truly representative of the test object undergoing examination. Where charge buildup on the insulating membrane is a problem, because of the resistivity of the particular membrane involved, the charge can be neutralized between exposure by means of any suitable device such as an alpha-emitter.

To insure that the initial resistivity is maintained within the desired range, the membrane material chosen should be substantially nonmoisture sensitive such that absorption of additional quantities of water will not lower the resistivity to a point where it is unsuitable for use, as described herein.

Since most materials of suitable resistivity will retain surface charge that can cause artifacts due to charge disturbance, where the membrane contacts charged portions of the xeroradiographic plate, the plurality of point contacts, as

described herein, serve to hold essentially the entire membrane surface sufficiently spaced from the plate surface such that image degrading artifacts are not produced.

In use, the membrane will be positioned either in direct contact with the surface of the xeroradiographic plate or, in the automated xeroradiographic system described in the copending application Ser. No. 874,834, filed Nov. 7, 1969, the membrane will be stretched between and over supporting members in the cassette described in copending application Ser. No. 874,747, also filed Nov. 7, 1969, at a distance which will enable point contact, as described herein, of the membrane and the xeroradiographic plate during imaging exposure. Accordingly, a membrane utilized in an automated system of the type described in said copending application should be sufficiently pliable or flexible either permit mounting thereof in the cassette, for example, as shown in FIG. 5 of the latter application.

A suitable membrane material which has prevented image degradation under conditions which caused degradation with the materials suggested by Lehmann, is a matte finish 0.002 inch thick drafting film of polyethylene terephthlate. Additionally, textured material with either a random or regular point contact pattern on approximately 0.001 inch centers should be effective in eliminating image degradation while at the same time preventing the formation of visible image artifacts superimposed over the latent image of the test object. Such a material could be prepared, for example, by evaporating a very fine pattern of point contact-defining material, such as aluminum or a plastic material, onto a suitable plastic film having the desired properties. Since most sheet material does not have the required surface, the textured nature thereof can be obtained by suitably texturing the surface desired to be positioned adjacent the xeroradiogarphic plate by, for exam ple, chemical etching, embossing, finely particulate mechanical etching (i.e., blasting), vacuum forming, etc. Suitable materials which can be textured to provide the desired point contact surface include polyvinyl chloride, nylon, polyvinyl acetate, polytetrafluoroethylene etc.

DESCRIPTION OF SPECIFIC EMBODIMENTS The following Examples are given to enable those skilled in the art to more clearly understand and practice the present invention. They should not be considered as a limitation upon the scope of the invention but merely as being illustrative thereof.

In the following Examples, a xeroradiographic plate comprising a 130 micron thick selenium layer on a mil thick aluminum substrate is given a uniform positive electrostatic charge and exposed to an X-ray pattern created by passing X- rays through a 6 inch by 6 inch test object comprising a onehalf inch thick cross section of breast tissue encapsulated in a 1 inch thick plexiglas block. A U-shaped cassette simulator, fabricated from the phenylene oxide related Noryl (a product of the General Electric Company), with the membrane material stretched across the open part of the U is positioned with the membrane in direct contact with the xeroradiographic plate. The cassette simulator is constructed to provide a 200 mil air gap above the membrane (i.e., between the inside, parallel surfaces of the membrane and the cassette simulator). The upper portion of the cassette simulator parallel to the stretched membrane is 100 mils thick and it is upon the upper surface thereof that the aforementioned test object is placed. X-ray radiation from a tungsten-molybdenum anode X-ray tube (Continental X-Ray Company), spaced 30 inches from the test object, passes through the test object, the test object supporting portion of the cassette simulator, and the stretched membrane before it exposes the xeroradiographic plate. The latent image created thereby is developed with conventional Xerox Type V blue toner in a powder cloud development apparatus described in application Ser. No. 874,746, filed Nov. 7, 1969. The developed images are then visually examined for the presence or absence of artifacts, imperfections, etc.

Examples l and II The selenium plate was charged to a potential of +1400 volts. A 2 mil thick polyethylene terephthlate drafting film was stretched across the cassette simulator and placed in direct contact with the xeroradiographic plate. The test object was exposed to a 40 k.v.p., 66 m.a.s. X-ray source at the 30 inch target-object distance. The temperature during exposure was on the order of 657SF. with the relative humidity approximately 20 percent. There was no evidence of image degradation in the resultant developed image. The procedure was repeated approximately 2-3 minutes later without the 2 mil polyethylene terephthlate drafting film. Without the film, there was evidence, as determined by visual examination, of image degradation.

Examples Ill and IV The procedures of Examples 1 and II were repeated with the exposure conditions changed to 45 k.v.p. and 50 m.a.s.. With the polyethylene terephthlate film in place, there was no visible evidence of image degradation while without the film, image degradation was readily perceptible.

Examples V and VI The procedures of Examples 1 and II were repeated with the exposure conditions changed to 50 k.v.p. and 40 m.a.s.. With the polyethylene terephthlate film in place, there was no visible indication of image degradation while without the film, image degradation was readily perceptible.

Examples VII and Vlll The procedures of Examples 1 and II were repeated with the exposure conditions changed to 55 k.v.p. and 33 m.a.s.. With the polyethylene terephthlate film in place, there was no visible indication of image degradation while without the film, image degradation was readily perceptible.

Examples IX and X The procedures of Examples I and II were repeated with the exposure conditions changed to 60 k.v.p. and 20 m.a.s.. With the polyethylene terephthlate film in place, there was no visible indication of image degradation while without the film, image degradation was readily perceptible.

Example XI Examples X11 and Xlll The procedure of Example XI was repeated twice with similar results. After exposure, a potential of +100 volts was detected on the polyethylene terephthlate film but this did not have an adverse effect on image quality.

Example XIV The procedure of Example Xl was repeated with the test object removed. No artifacts were found when the resultant latent image on the xeroradiographic plate was developed.

Examples XV and XVl The procedure of Example XI was repeated twice using 3 mil thick cellulose acetate having a 100 mil air gap between the upper surface thereof and the adjacent surface of the test object support. The first exposure produced a satisfactory image without evidence of image degradation. The second exposure resulted in a developed image having visibly perceptible image degradation.

Example XVll The procedure of Example XV was repeated using 2 mil thick cellulose acetate. Image degradation of the developed image was readily perceptible.

The xeroradiographic plate utilized in Examples XVXVll was uniformly charged to a potential of +1400 volts'and developed without exposure. This procedure was repeated a second time. No artifacts were produced in either developed image, normally called a dark dusting". This is an indication that there were no imperfections in the xeroradiographic plate utilized in Examples XVXVll.

While the present invention has been described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, apparatus or process, to the spirit of the present invention without departing from its essential teachings.

What I claim is:

1. In a xeroradiographic process wherein a xeroradiographic plate including an Xray sensitive photoconductive layer is exposed to an X-ray radiation pattern which has passed through a test object, the improvement adapted to eliminate the adverse effects of ion generation adjacent the xeroradiographic plate surface such as might cause latent electrostatic image disturbance; said improvement comprising (a) positioning in direct contact with the charge sensitized surface of said xeroradiographic plate a thin, insulating membrane having a resistivity at least equal to the dark resistivity of said photoconductor layer or the bulk resistivity of an insulating coating overlying said photoconductive layer, said insulating membrane further having a matte surface having a plurality of point contacts which contact said charge sensitized surface of said xeroradiographic plate, said point contacts being of limited surface area such that should discharge of the electrostatic charge on said charge sensitized surface occur, the discharged areas caused thereby are below the limit of the eyes resolving power such that said discharged areas cannot be detected during subsequent visual examination of the resultant developed image, the distance between adjacent point contacts and the distance between said charge sensitized surface and the adjacent surface of the bulk of said insulating membrane being less than the ionizing means free path of an X-ray generated electro in air, and (b) exposing said xeroradiographic plate through said insulating membrane to said X-ray radiation pattern while said matte surface of said insulating membrane is in contact with said charge sensitized surface of said xeroradiographic plate.

2. The process of claim 1 wherein the resistivity of said insulating membrane is on the order of about 10 to about 10 ohm-cm.

3. The process of claim 1 wherein the resistivity of said insulating membrane is on the order of about 10 to about 10 ohm-cm.

4. The process of claim 1 wherein said insulating membrane is nonmoisture sensitive.

5. The process of claim 1 wherein the individual surface area of substantially all of said point contacts is on the order of a circle having a diameter of about 0.001 to about 0.002 inch.

6. The process of claim 1 wherein the individual surface area of substantially all of contacting point contacts is on the order of a circle having a diameter not greater than about 0.002 inch.

7. The process of claim 1 wherein the total surface area of the point contacts contacting said xeroradiographic plate is on the order of about 0.1 percent to about 1 percent of the total surface area of the charge sensitized surface of said photoconductive layer.

8. The process of claim 1 wherein the distance between adjacent point contacts and the distance between said xeroradiographic plate surface and the adjacent surface of the bulk of said insulating membrane are each on the order of about 0.001 to about 0.003 inch.

9. The process of claim 1 wherein said insulating membrane has a sufficiently uniform internal structure such that said insulating membrane does not differentially absorb imaging radiation across the exposed surface thereof, whereby the resultant latent electrostatic image is truly representative of the test object undergoing examination.

10. The process of claim 1 wherein said insulating membrane is sufficiently flexible to permit mounting thereof on support means adjacent the charge sensitized surface of said xeroradiographic plate, said membrane further having sufficient strength to resist broadening of the surface area of said point contacts under compressive stress.

11. The process of claim 1 wherein said insulating membrane comprises a plastic material textured to provide said matte surface.

12. The process of claim 1 wherein said insulating membrane comprises a material having a plurality of individual fibers extending from at least one surface thereof, the ends of said fibers functioning as said point contacts.

13. The process of claim 1 wherein said insulating membrane comprises an insulating substrate having said plurality of point contacts deposited thereon.

14. The process of claim 1 wherein said insulating membrane comprises textured polyethylene terephthlate.

15. The process of claim 14 wherein said polyethylene terephthlate membrane is approximately 2 mils thick.

16. The process of claim 1 further including the step of developing the latent electrostatic image resulting from said exposure step into a corresponding reproduction thereof suitable for visual examination, said developed reproduction being characterized by the absence of image degradation normally associated with the positioning of a flat insulating membrane in contact with the charge sensitized surface during said exposure.

17. In a xeroradiographic process wherein a xeroradiographic plate including an X-ray sensitive photoconductive layer is exposed to an X-ray radiation pattern created by passing X-rays through a test object, the improvement adapted to eliminate the adverse effects of ion generation adjacent the xeroradiographic plate surface such as might cause latent electrostatic image disturbance; said improvement comprising (a) positioning in direct contact with the charge sensitized surface of said xeroradiographic plate a thin, nonmoisture sensitive, nondifferential absorbing insulating membrane having a resistivity on the order of about l0 to about 10 ohm-cm, said insulating membrane further having a matte surface having a polarity of point contacts which contact said charge sensitized surface of said xeroradiographic plate, the individual surface area of substantially all of said point contacts being on the order of a circle having a diameter not greater than about 0.002 inch, the total surface area of said point contacts and contacting said charge sensitized surface being on the order of about 0.1 percent to about 1 percent of the total surface area of said charge sensitized surface, the distance between adjacent point contacts and the distance between said charge sensitized surface and adjacent surface of the bulk of said insulating membrane being on the order of about 0.00] to about 0.003 inch, and (b) exposing said xeroradiographic plate through said insulating membrane to said X-ray radiation pattern while said matte surface of said insulating membrane is in contact with said charge sensitized surface of said xeroradiographic plate.

18. The process of claim 17 wherein said insulating membrane is sufficiently flexible to permit mounting thereof on support means adjacent the charge sensitized surface of said xeroradiographic plate, said membrane further having sufficient strength to resist broadening of the surface area of said point contacts under the influence of electrostatically induced com ressive forces.

1 The process of claim 17 wherein said insulating membrane comprises a plastic material textured to provide said matte surface.

20. The process of claim 17 wherein said insulating membrane comprises textured polyethylene terephthlate.

21. The process of claim 20 wherein said polyethylene terephthlate membrane is approximately 2 mils thick.

22. The process of claim 17 further including the step of developing the latent electrostatic image resulting from said exposure step into a corresponding reproduction thereof suitable for visual examination, said developed reproduction being characterized by the absence of image degradation associated with the positioning of a flat insulating membrane in contact with the charge sensitized surface during said exposure.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORREC [ION Patent No. 3 584, 2155 Dated June 8, l)? l Invent0r(5) William J. Kearns et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 50, "means" should read -mean.

On line 5l, "electro" should read -electron.

Column 8, line 7, "polarity should read -plurality. On line 12, "and" should be deleted.

On line 16, after "and" the term the-- should be inserted.

Signed auu sealed Lhi .er rind day oi November? 31"! A v fattest:

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2. The process of claim 1 wherein the resistivity of said insulating membrane is on the order of about 1012 to about 1018 ohm-cm.
 3. The process of claim 1 wherein the resistivity of said insulating membrane is on the order of about 1012 to about 1015 ohm-cm.
 4. The process of claim 1 wherein said insulating membrane is nonmoisture sensitive.
 5. The process of claim 1 wherein the individual surface area of substantially all of said point contacts is on the order of a circle having a diameter of about 0.001 to about 0.002 inch.
 6. The process of claim 1 wherein the individual surface area of substantially all of contacting point contacts is on the order of a circle having a diameter not greater than about 0.002 inch.
 7. The process of claim 1 wherein the total surface area of the point contacts contacting said xeroradiographic plate is on the order of about 0.1 percent to about 1 percent of the total surface area of the charge sensitized surface of said photoconductive layer.
 8. The process of claim 1 wherein the distance between adjacent point contacts and the distance between said xeroradiographic plate surface and the adjacent surface of the bulk of said insulating membrane are each on the order of about 0.001 to about 0.003 inch.
 9. The process of claim 1 wherein said insulating membrane has a sufficiently uniform internal structure such that said insulating membrane does not differentially absorb imaging radiation across the exposed surface thereof, whereby the resultant latent electrostatic image is truly representative of the test object undergoing examination.
 10. The process of claim 1 wherein said insulating membrane is sufficiently flexible to permit mounting thereof on support means adjacent the charge sensitized surface of said xeroradiographic plate, said membrane further having sufficient strength to resist broadening of the surface area of said point contacts under compressive stress.
 11. The process of claim 1 wherein said insulating membrane comprises a plastic material textured to provide said matte surface.
 12. The process of claim 1 wherein said insulating membrane comprises a material having a plurality of individual fibers extending from at least one surface thereof, the ends of said fibers functioning as said point contacts.
 13. The process of claim 1 wherein said insulating membrane comprises an insulating substrate having said plurality of point contacts deposited thereon.
 14. The process of claim 1 wherein said insulating membrane comprises textured polyethylene terephthlate.
 15. The process of claim 14 wherein said polyethylene terephthlate membrane is approximately 2 mils thick.
 16. The process of claim 1 further including the step of developing the latent electrostatic image resulting from said exposure step into a corresponding reproduction thereof suitable for visual examination, said developed reproduction being characterized by the absence of image degradation normally associated with the positioning of a flat insulating membrane in contact with the charge sensitized surface during said exposure.
 17. In a xeroradiographic process wherein a xeroradiographic plate including an X-ray sensitive photoconductive layer is exposed to an X-ray radiation pattern created by passing X-rays through a test object, the improvement adapted to eliminate the adverse effects of ion generation adjacent the xeroradiographic plate surface such as might cause latent electrostatic image disturbance; said improvement comprising (a) positioning in direct contact with the charge sensitized surface of said xeroradiographic plate a thin, nonmoisture sensitive, nondifferential absorbing insulating membrane having a resistivity on the order of about 1012 to about 1018 ohm-cm, said insulating membrane further having a matte surface having a polarity Of point contacts which contact said charge sensitized surface of said xeroradiographic plate, the individual surface area of substantially all of said point contacts being on the order of a circle having a diameter not greater than about 0.002 inch, the total surface area of said point contacts and contacting said charge sensitized surface being on the order of about 0.1 percent to about 1 percent of the total surface area of said charge sensitized surface, the distance between adjacent point contacts and the distance between said charge sensitized surface and adjacent surface of the bulk of said insulating membrane being on the order of about 0.001 to about 0.003 inch, and (b) exposing said xeroradiographic plate through said insulating membrane to said X-ray radiation pattern while said matte surface of said insulating membrane is in contact with said charge sensitized surface of said xeroradiographic plate.
 18. The process of claim 17 wherein said insulating membrane is sufficiently flexible to permit mounting thereof on support means adjacent the charge sensitized surface of said xeroradiographic plate, said membrane further having sufficient strength to resist broadening of the surface area of said point contacts under the influence of electrostatically induced compressive forces.
 19. The process of claim 17 wherein said insulating membrane comprises a plastic material textured to provide said matte surface.
 20. The process of claim 17 wherein said insulating membrane comprises textured polyethylene terephthlate.
 21. The process of claim 20 wherein said polyethylene terephthlate membrane is approximately 2 mils thick.
 22. The process of claim 17 further including the step of developing the latent electrostatic image resulting from said exposure step into a corresponding reproduction thereof suitable for visual examination, said developed reproduction being characterized by the absence of image degradation associated with the positioning of a flat insulating membrane in contact with the charge sensitized surface during said exposure. 